Grinding in tube-mills.



0. SCHULTZ. GRINDING IN TUBE MILLS. APPLICATIVON FILED FEB. x2. 1914.

.wlu l mented May 8,1917.

M M@ v C E Q 90 D im f MA? Mmm/ HS E@ STA H lPATlENT @FERR OSCAR SCH'ULTZ, OF BROOKLYN, NEW YORK, ASSIGNOR TO F. L. SMIDTH & CO., OF NEW YORK, N. Y., A CORPORATION 0F NEW JERSEY.

GRINDING IN TUBE-MILLS.

Specification of Letters Patent.

Patentedlttay 8, 1917.

Application filed February 12, 1914. Serial No. 818,186.

To f/ZZ whom it ymay concern.'

Be it known that l, Oscar: SCHULT'II, a subject of the King of Denmark, residing in the borough of Brooklyn of the city of New York, in the county of Kings, in the State of New York, have invented certain new and useful Improvements in Grinding in Tube-Mills, of which the following is a specific-ation, reference being had to the accompanying drawings, which form a part hereof.

This invent-ion relates to the fine grinding of various substances, such as cement clinker, coal for use in rotary kilns, etc., in rotary drums which are charged with grinding bodies lsuch as flint pebbles or metallic grinding bodies. The common practice in this art is to rotate the grinding drum at a speed in revolutions per minute which is equal to the quotient of 30 divided by the square root of the diameter of the drum expressed in meters, the speed being usually expressed by the formula R. P. M. da

The charge of grinding bodies is usually forty-five pounds per cubic foot of the volume of the drinn when flint pebbles of a specific gravity of approximately 2.6 are used, or one hundred and ten pounds per cubic foot of the volume of the drum when metallic .grinding bodies of a specific gravity of approximately 7.5 are used. Under such conditions of charge and speed it is probable that the surface of the mass within the drum, including both the grinding bodies and the material being ground, with respect to a horizontal plane. is made constantly to exceed the angle of repose of the mass so that the grinding bodies, whether of one kind or another. slip and slide and tumble or cascade down the face of the mass. Such action may be true of the portion of the mass which lies near the face as well as of the face of the mass itself. 1t has been found heretofore that a greatly increased efficiency can be secured in the operation of such grinding mills by reducing the charge of grinding bodies to about one-half, by weight per cubic foot, of the charge usually employed heretofore, the weight of the material being ground and in the drum at any one time being also reduced somewhat, while the speed of rotation of the drum is considerably increased, so that the grinding bodies, instead of having a rolling and rubbing action as heretofore, fall freely, under the combined action of momentum and gravity, through a greater' or less distance within the drum and therefore have a percussive action, which is found to be very highly effective in pulverizing the material in the drum. This method of grinding in tube mills is fully described and broadly claimed in a co-pending'application filed by Meyer J. Davidsen, Ser. No. 7 83,470 on August 7, 1913. and owned by the assignee of the present application. Grinding under the new conditions of weight of charge and speed of rotation can be carried on with flint pebbles but the economical results are more satisfactory with metallic grinding bodies since the flint pebbles are liable to be fractured by the percussive action. For relatively coarser grinding, as is well understood, pebbles are more satisfactory than small metallic grinding bodies, and for finer grinding small metallic grinding bodies are more satisfactory than flint pebbles. As a natural result, therefore, the preliminary grinding with flint pebbles is carried on in one grinding mill and the final grinding with small metallic grinding bodies is carried on in another grinding mill. The employment of two mills is accompanied by correspondingly large cost of plant, of power and of labor and by incidental losses. lt is impossible to realize the advantages of relatively high speed rotation in the operation of grinding in an ordinary mill, of uniform diameter, even if a partition be placed therein and the inlet chamber be charged with liint pebbles and the delivery chamber with metallic grinding bodies, since the pcripheral speed of the two chambers is the same and. for the reason already suggested, the flint pebbles would be fractured if the mill were driven at the higher speed which can be employed to advantage with metallic grinding bodies.

1t is the object of this invention to permit both the preliminary grinding with flint pebbles and the final grinding with metallic grinding bodies to be accomplished at one operation and in one mill and at the same time to enable the greater eliiciency possible through grinding with metallic grinding bodies at relatively high speed to be realized. In accordance with the invention the grinding with flint pebbles is carried on in one part of the mill which has a relatively smaller diameter and therefore a relatively slow peripheral speed, and the grinding with metallic grinding bodies is carried on in an immediately succeeding part of the same mill which has a larger diameter and therefore a greater peripheral speed so that the flint pebbles have the cascading action and are not broken up with excessive rapidity, and the metallic grinding bodies have such speed that under the resultant of momentum and gravit-y they fall freely and have a percussive action, this method being described fully in the application above identified. n

The invention will be more fully explained hereinafter with reference to the accompanying drawings in which it is illustrated and in which- Figure l is a view in longitudinal section of a tube mill constructed in accordance with the invention.

Fig. 2 is a somewhat diagrammatic view in section on the plane indicated by the line 2-2 of Fig. 1.

',Fig. 3 is a similar view in section on the plane indicated by the line 3-3 of Fig. 1.

The drum in which the grinding is carried on may be constructed in any suitable manner and mounted so as to be rotated by suit-v able means. It may have in general any desired or suitable dimensions. It comprises, as indicated in the drawings an inlet chamber a, internally, at least, of relatively small diameter, and a discharge chamber b, internally, at least, of relatively large diameter. The two chambers are. provided respectively with inlet and discharge openings and are separated by a perforated partition c. The two parts of the drum are axially alined and rigidly connected so that they rotate as one or with the same angular velocity.

The inlet or feed chamber a is charged with flint pebbles, and the charge may be the usual charge of about forty-five pounds per cubic foot of the volume of the chamber The speed of rotation of the chamber a may also be expressed by the usual formula, given above,

R.P.M.= 45

Since the two chambersa and b are parts La V515 This gives That is, the ratio of the diameters of the two chambers should be about as 302 to 452.

The relative lengths of the two chambers a and 7) may be varied according to circumstances, the length of the chamber a being preferably from two to three times the length of the chamber b.

In the operation of grinding in a mill constructed as described and operated with flint pebbles in the inlet chamber and with small metallic grinding bodies in the discharge chamber, the chambers being approximately of the relative diameters described and the whole mill being rotated at a speed expressed by the formula in which d is the diameter of the inlet chamber in meters, or by the formula in which D is the diameter of the discharge chamber in meters, the flint pebbles in the inlet chamber will be operated under the normal conditions established by the usual tube mill practice, whereas the small metallic grinding bodies in the discharge chamber will be operated under the conditions of relatively higher speed previously referred to. The flint pebbles will roll and slide over one another, being supported always in part by the shell of the mill, whereas the small metallic grinding bodies, with the materia-l will be of relatively small combined bulk and under the influence of the greater peripheral speed of the discharge chamber, will be carried by centrifugal force in a relatively thin layer to a point where they are mechanically unsupported against the action of gravity and the layer will break so that the mass will therefore describe a parabolic curve within the mill, as indicated in Fig. 3, and will have a percussive action.

It will be understood that the ratio of the diameters of the two parts of the mill, indicated above, is only approximate and may be varied somewhat under dierent conditions of operation. Thus for dry grinding the ratio would be expressed by the formula while for wet grinding the ratio would be expressed by the formula It will be understood that even these ratios may be varied somewhat under different conditions of operation.

The general result of operation in the manner and with the mill described will be the performance of both preliminary grinding and final grinding in one mill and in one continuous operation and with a considerable saving in horse power efected through the nal grinding with metallic grinding bodies at relatively high speed and a consequent increase in efficiency.

I elaim as my invention A tube mill for grinding with grinding bodies comprising an inlet chamber o'F. relatively small diameter and an outlet chamber of relatively larger diameter, a perforated partition between said chambers, flint pebbles in the inlet chamber and small metallic grinding bodies in the outlet chamber, the relative diameters of said chambers being such that the peripheral speeds cause the flint pebbles to have a cascading action while the metallic grinding bodies are carried by centrifugal force in a thin, whirling layer along an uninterrupted endless path and confined to such path so that a Jforward portion of the bodies falls upon a rear portion of the same.

This specification signed and witnessed this tenth day of February, A. D. 1914:.

OSCAR SCHULTZ.

WORTHINGTON CAMPBELL. 

